With a recent trend toward higher integration and higher density in semiconductor devices, circuit interconnects become finer and finer and the number of levels in multilayer interconnects is increasing. In the fabrication process of the multilayer interconnects with finer circuit, as the number of interconnect levels increases, film coverage of step geometry (or step coverage) is lowered in thin film formation because surface steps grow while following surface irregularities on a lower layer. Therefore, in order to fabricate the multilayer interconnects, it is necessary to improve the step coverage and planarize the surface. It is also necessary to planarize semiconductor device surfaces so that irregularity steps formed thereon fall within a depth of focus in optical lithography. This is because finer optical lithography entails shallower depth of focus.
Accordingly, the planarization of the semiconductor device surfaces is becoming more important in the fabrication process of the semiconductor devices. Chemical mechanical polishing (CMP) is the most important technique in the surface planarization. This chemical mechanical polishing is a process of polishing a wafer by bringing the wafer into sliding contact with a polishing surface of a polishing pad while supplying a polishing liquid containing abrasive grains, such as silica (SiO2), onto the polishing surface.
A polishing apparatus for performing CMP has a polishing table that supports the polishing pad thereon, and a substrate holder, which is called a top ring or a polishing head, for holding a wafer. When the wafer is polished using such a polishing apparatus, the substrate holder holds the wafer and presses it against the polishing surface of the polishing pad at a predetermined pressure, while the polishing table and the substrate holder are moved relative to each other to bring the wafer into sliding contact with the polishing surface to thereby polish a surface of the wafer.
As the types of semiconductor devices have been increasing dramatically in recent years, there has been a demand for polishing films having different film-thickness distributions for devices or CMP processes (e.g., an oxide film polishing process and a metal film polishing process). The reason of this is that each wafer has a different initial film-thickness distribution because a film-forming process, which is performed prior to the CMP process, varies depending on the type of film.
Generally, a wafer is required to have a uniform film-thickness distribution over an entire surface of the wafer. In a case where a wafer has different initial film thicknesses along a radial direction of the wafer, it is possible to control polishing rates (which are also referred to as removal rates) along the radial direction of the wafer by regulating pressures in a plurality of concentric pressure chambers which are formed by an elastic membrane in contact with the wafer, as shown in Japanese laid-open patent publication No. 2006-324413.
An initial film thickness of a wafer, to be polished, may vary along a circumferential direction of the wafer due to a characteristic of a film-forming device. In particular, the initial film thickness in a peripheral portion of the wafer tends to vary along the circumferential direction. In order to reduce such a variation in the film thickness, there have been proposed several polishing apparatuses. For example, Japanese laid-open patent publication No. 2002-079454 discloses a polishing apparatus which uses an annular piston divided into several pistons arranged along a circumferential direction of a wafer. Further, Japanese laid-open patent publication No. 2006-324413 discloses a substrate holder which has a pressure chamber for pressing a retainer ring. This pressure chamber is divided into several chambers arranged along a circumferential direction.
However, these polishing apparatuses use the divided pressing elements arranged along the circumferential direction of the wafer, and therefore the substrate holder entails an extremely complicated structure. Moreover, a pressing force may vary between the divided pressing elements. For example, in the Japanese laid-open patent publication No. 2006-324413, there may be a variation in expansion and contraction of the divided pressure chambers. Further, in the Japanese laid-open patent publication No. 2002-079454, a sliding resistance, which is generated when each piston is lowered, may vary between the pistons.
In particular, the substrate holder disclosed in the Japanese laid-open patent publication No. 2002-079454 has a plurality of divided rigid pistons, which are arranged along the circumferential direction of the wafer and press the wafer. With this structure, the polishing rate of the wafer may be discontinuous in the circumferential direction. Additionally, a variation in flatness of the pistons may result in a variation in polishing rate of the wafer.